Varistor selection formula

Varistors are widely used in household appliances and other electronic products, and they have overvoltage protection, lightning protection, suppression of inrush current, absorption of spikes, clipping, high voltage arc extinguishing, noise reduction, and protection of semiconductor components. Wait. What is the selection formula for varistors? Let's take a look at Xiaobian.

In general, varistors are used in parallel with protected devices or devices. Under normal conditions, the dc or ac voltage across the varistor should be lower than the nominal voltage, even when the power supply fluctuation is worst, it should not be higher than the continuous operating voltage selected in the rated value. The continuous operating voltage value The corresponding nominal voltage value is the selected value. For the application of overvoltage protection, the varistor voltage value VmA should be greater than the actual circuit voltage value, generally use the following formula to select:

Where: a is the circuit voltage fluctuation coefficient, generally take 1.2; v is the circuit DC operating voltage (AC is the effective value); b is the varistor voltage error, generally take 0.85; c is the aging factor of the component, generally take 0.9; The actual value of VmA calculated in this way is 1.5 times the DC operating voltage. Considering the peak in the AC state, the calculation result should be expanded by 1.414 times.

When choosing, you must also pay attention:

1) It must be ensured that the continuous operating voltage will not exceed the permissible value when the voltage fluctuates. Otherwise, the service life of the varistor will be shortened.

2) When a varistor is used between the power line and the ground, the line-to-ground voltage may rise due to a poor grounding. Therefore, a varistor having a higher nominal voltage than that used between the line and the line is usually used.

3) The surge current absorbed by the varistor should be less than the product's through-flow.